A man on death row in South Carolina has until 6 September to decide how he would prefer to be executed by the state.

South Carolina’s prisons director has declared the state’s supply of a lethal injection drug acceptable and said its electric chair was tested two months ago and its firing squad has the ammunition and training to carry out its first execution next month in more than 13 years, if needed.

Corrections director Bryan Stirling was ordered by the state supreme court to submit a sworn statement to the lawyer for Freddie Owens certifying that all three methods of putting a prisoner to death are available for his scheduled 20 September execution.

Owens’s lawyers have said they will review the statement, and if they do not think it is adequate, they will ask the state supreme court or federal judges to consider it.

Owens, 46, was sentenced in 1999 for shooting and killing Irene Graves, a gas station employee in Greenville during a robbery in 1997.

Owens has until 6 September to decide how he wants to die, and he signed his power of attorney over to his lawyer, Emily Paavola, to make that decision for him. The state supreme court has agreed to a request from the prison system to see if that is allowed under South Carolina law.

The power of attorney was signed under the name Khalil Divine Black Sun Allah. Owens changed his name in prison but goes by his old name in his legal hearings with the state to avoid confusion.

In the sworn statement, Stirling said technicians at the state Law Enforcement Division laboratory tested two vials of the sedative pentobarbital, which the state plans to use for lethal injections.

Stirling released no other details about the drugs under the guidelines of the state’s new shield law, which keeps secret the name of the supplier of the drug and anyone who helps carry out the execution. The law’s passage in 2023 also helped restart executions so the state could buy pentobarbital and keep the supplier private.

The state’s electric chair, built in 1912, was tested 25 June and found to be working properly, Stirling wrote, without providing additional details.

The firing squad, allowed by a 2021 law, has the guns, ammunition and training it needs, Stirling wrote. Three volunteers have been trained to fire at a target placed on the heart from 15ft away.

In South Carolina, the governor, Henry McMaster, has the ability to grant clemency to an individual on death row. However, no governor has done so in the state’s past 43 executions, the Associated Press reports, adding that McMaster told reporters on Tuesday that he would only announce his decision minutes before the execution when prison officials dial his office from the death chamber.

“When the rule of law has been followed, there really is only one answer,” McMaster said.

In response to McMaster’s comments, Hillary Taylor, executive director of South Carolinians for Alternatives to the Death Penalty, said: “Just because something is law does not mean that it is justice. Justice would actually be making sure that we interrupt and prevent violence from happening in the first place, not execute somebody who’s been on death row for decades.”

Earlier this week, Taylor’s organization circulated a petition for clemency from McMaster, which has garnered more than 1,300 signatures.

“Khalil Allah … is somebody who experienced profound trauma as a child [and] as an adolescent, both in terms of his family of origin and also the South Carolina judicial system … We do not believe that Khalil should be executed for things that are our fault as a society,” said Taylor.

Calling the death penalty “cheap justice”, Taylor added: “We would literally be better spending money on protecting children and creating more victim services for people who are experiencing violence and harm … The death penalty doesn’t do any of that.”

Taylor also pushed back on the conservative state’s so-called “pro-life” stance on reproductive rights, particularly its six-week abortion ban amid the backdrop of its death penalty law.

“There are government officials in South Carolina who like to brag that South Carolina is an extremely pro-life state, and if indeed all lives matter, then Khalil’s life especially matters because of the ways that he has been let down. He does not deserve to be cut off from this life. He deserves to live,” she said.

The South Carolina chapter of the American Civil Liberties Union (ACLU) has also condemned the death penalty, with executive director Jace Woodrum saying: “The state is preparing to kill one of our neighbors under a shroud of secrecy. Politicians and prison officials have not only hidden key details about all three execution methods, but they have refused to allow media interviews with people held on death row as they make their last pleas for clemency. We are joining faith leaders, civil rights leaders, and people of good conscience around our state calling on governor McMaster to stop this execution and all others.”

The Guardian has asked the governor’s office for comment.

The Associated Press contributed reporting.